test: add 3xx-redirect failure and no-credential-header/URL checks

Extends the newsletter suite with two boundary cases: redirects (301/302/307/
308) must be treated as failures with the user-safe error, and the POST must
carry no credential of any kind — no api-key/auth-token/cookie headers, and no
token/secret in the body or URL.

tests/newsletter.test.js

@@ -197,7 +197,20 @@ test("submitNewsletterSignup POSTs the email with no Authorization header or cre
     !Object.keys(init.headers).some((h) => h.toLowerCase() === "authorization"),
     "request must not carry an Authorization header",
   );
+  // No credential of any kind: no api-key/auth-token headers, and no token or
+  // secret in the body or URL either.
+  for (const header of Object.keys(init.headers)) {
+    const lower = header.toLowerCase();
+    assert.ok(
+      !["x-api-key", "x-auth-token", "x-access-token", "cookie"].includes(lower),
+      `request must not carry credential header ${header}`,
+    );
+  }
   assert.deepEqual(JSON.parse(init.body), { email: "ada@example.com" });
+  assert.ok(!url.includes("token"));
+  assert.ok(!url.includes("key"));
+  assert.ok(!url.includes("secret"));
+  assert.ok(!JSON.stringify(init.body).includes("token"));
 });
 
 test("submitNewsletterSignup defaults to the configured endpoint", async () => {
@@ -285,6 +298,18 @@ test("a network failure shows the same user-safe error", async () => {
   assert.equal(result.message, NEWSLETTER_ERROR_MESSAGE);
 });
 
+test("a redirect (3xx) is treated as a failure with the same user-safe error", async () => {
+  for (const status of [301, 302, 307, 308]) {
+    const fetchImpl = fakeFetch({ ok: false, status, text: async () => "redirecting" });
+    const result = await submitNewsletterSignup("ada@example.com", {
+      endpoint: "https://api.example.com/newsletter",
+      fetchImpl,
+    });
+    assert.equal(result.ok, false);
+    assert.equal(result.message, NEWSLETTER_ERROR_MESSAGE);
+  }
+});
+
 test("an invalid email fails fast with a user-safe error and no network call", async () => {
   const fetchImpl = fakeFetch({ ok: true });
   const result = await submitNewsletterSignup("not-an-email", {